This invention relates to aircraft blades and in particular, to a new and improved method and apparatus for continuously detecting cracks in propeller and helicopter rotor blades.
Aircraft blades are subjected to severe stress and occasionally develop minute cracks. It is of critical importance that the crack in the blade be detected at an early time so that the blade may be replaced preventing an inflight accident. A wide variety of methods are available for detecting cracks, including the making of x-ray pictures and the magnetic flux techniques. However these require considerable equipment and can only be performed when the aircraft is at rest.
Another system presently in use provides a pressure differential in a sealed hollow blade, with the interior blade pressure being either above or below atmospheric. A pressure sensor is mounted on the blade and provides a visual indication of the pressure differential, with a drop in the differential indicating leakage due to a crack. This type of device provides for blade integrity measurement without requiring removal of the blade from the aircraft. However the visual inspection can be performed only when the aircraft is on the ground. It has been suggested that the blade internal pressure be transmitted to an indicator in the cockpit while the aircraft is in flight, utilizing a set of slip rings at the rotating hub for information transmission. However slip ring systems present a number of problems. They are susceptible to dirt and grease, they are difficult to install and maintain, and they increase the cost and complexity of an already complex rotor shaft system.
It is an object of the present invention to provide a new and improved method and apparatus for continuously indicating the condition of hollow aircraft blades while the aircraft is in flight. A further object is to provide such method and apparatus which does not require any connection between the rotating blade and the remainder of the aircraft and which can be utilized to give go-no go indications for safe and warning conditions, and/or proportional type indications as a measure of pressure differential.